In linguistics I learned that French stress at the lexical level is always final except syllabes facultatives. Word-initial stress would thus be prosodic stress. I do hear a lot of this when listening to French. Even in the first 5 seconds of this audiobook! « Le petit prince. » I think that stress marks a "block of interest". It occurs often with that function: 0:36 magnifique, 0:40 histoires, 0:51 sans, 1:08 premier, 1:10 dessin... Are the ones you hear of a very different nature?
– Luke SawczakMay 10 '18 at 1:48

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@LukeSawczak I only did a very quick and biased survey (listened to a few different radio programs). This would probably have more prosodic stress than ordinary speech, and indeed there seemed to be a weak correlation between initial stress and emphasis. But there's more to it than this. Some speakers used initial stress in random-sounding patterns. What seems to be well-known in linguistics doesn't match reality — I think it describes the majority of speakers/circumstances, but it's not a universal rule by far.
– Gilles♦May 10 '18 at 10:10

Having now spent a bit more time actively listening for this on the radio, YouTube, podcasts, etc., I'm tempted to agree that either this "emphatic" usage is so ubiquitous that it has no particular meaning, or it's not the only reason the stress falls on the first syllable!
– Luke SawczakMay 19 '18 at 15:53